Polyisocyanates containing s-triazine units prepared by the reaction of melamine with excess quantities of aromatic diisocyanates containing isocyanate groups with differing reactivities are described in European Pat. No. 893 (corresponding to U.S. patent application Ser. No. 930,611, filed Aug. 3, 1978 and now U.S. Pat. No. 4,255,570, issued Mar. 10, 1981). The compounds described in that prior publication are solid substances which range in their solubility in organic media from difficult to solubilize to insoluble and which are very high melting and decompose in the melt. Due to their high melting point and their poor solubility or compatibility, the usefulness of the abovementioned polyisocyanates as reactive fillers in the production of polyurethanes by the isocyanate polyaddition process is strictly limited.
It was therefore an object of the present invention, starting from the known polyisocyanates containing s-triazine units, to develop fillers capable of being chemically fixed which do not have those disadvantages, but rather are capable of melting without decomposition and/or have improved compatibility with the starting materials used for the production of polyurethanes.